Dissecting the functional roles of neuronal circuits and their interaction is a crucial step in basic neuroscience and in all the biomedical field. Optogenetics is well-suited to this purpose since it allows us to study the functionality of neuronal networks on multiple scales in living organisms. This tool was recently used in a plethora of studies to investigate physiological neuronal circuit function in addition to dysfunctional or pathological conditions. Moreover, optogenetics is emerging as a crucial technique to develop new rehabilitative and therapeutic strategies for many neurodegenerative diseases in pre-clinical models. In this review, we discuss recent applications of optogenetics, starting from fundamental research to pre-clinical applications. Firstly, we described the fundamental components of optogenetics, from light-activated proteins to light delivery systems. Secondly, we showed its applications to study neuronal circuits in physiological or pathological conditions at the cortical and subcortical level, in vivo. Furthermore, the interesting findings achieved using optogenetics as a therapeutic and rehabilitative tool highlighted the potential of this technique for understanding and treating neurological diseases in pre-clinical models. Finally, we showed encouraging results recently obtained by applying optogenetics in human neuronal cells in-vitro.

Genetically encoded calcium indicators (GECIs) can be used to monitor the intracellular concentration of free calcium in populations of neurons. GECIs with long emission wavelengths are particularly attractive for deep tissue microscopy in vivo, and have the additional advantage of avoiding spectral overlap with commonly used neuronal actuators such as channelrhodopsin.The aim of this work is to evaluate the performances of four red-shifted GECIs (jRCaMP1a, jRCaMP1b, jRGECO1a, jRGECO1b) using both ex vivo and in vivo experimental techniques.Cortical neurons were infected with adeno-associated virus (AAV) expressing one of the red GECI variants. First we characterized the transfection ex vivo in terms of extension and intensity of the indicator. Next, we monitored the neuronal activity over the right cortical hemisphere of a jRCaMP1a-transfected mouse during a goal-directed motor task. To this aim, we combined wide-field fluorescence microscopy with a robotic device for simultaneous recording of cortical neuronal activity, force applied and forelimb position during task execution.Our results show that jRCaMP1a has sufficient sensitivity to monitor in vivo neuronal activity simultaneously over multiple functional areas, and can be successfully used to perform longitudinal imaging sessions in awake mice.

No abstract

Brain states, such as wake, sleep, or different depths of anesthesia are usually assessed using electrophysiological techniques, such as the local field potential (LFP) or the electroencephalogram (EEG), which are ideal signals for detecting activity patterns such as asynchronous or oscillatory activities. However, it is technically challenging to have these types of measures during calcium imaging recordings such as two-photon or wide-field techniques. Here, using simultaneous two-photon and LFP measurements, we demonstrate that despite the slower dynamics of the calcium signal, there is a high correlation between the LFP and two-photon signals taken from the neuropil outside neuronal somata. Moreover, we find the calcium signal to be systematically delayed from the LFP signal, and we use a model to show that the delay between the two signals is due to the physical distance between the recording sites. These results suggest that calcium signals alone can be used to detect activity patterns such as slow oscillations and ultimately assess the brain state and level of anesthesia.

Intracellular concentration of free calcium ions in neuronal populations can be longitudinally evaluated by using fluorescent protein indicators, called genetically encoded calcium indicators (GECIs). GECIs with long emission wavelengths are particularly attractive for deep tissue microscopy in vivo, and have the additional advantage of avoiding spectral overlap with commonly used neuronal actuators like Channelrhodopsin. Here we investigated the performances of selected red-shifted GECIs through an ex vivo characterization and in vivo imaging of cortical mouse activity during motor task execution. Cortical neurons were infected with adeno-associated virus (AAV) expressing one of the red GECI variants (jRCaMP1a, jRCaMP1b, jRGECO1a, jRGECO1b). First we characterized the transfection in terms of extension and intensity using wide-field fluorescence microscopy. Next, we used RCaMP1a to analyse the cortical neuronal activity during motor behaviour. To that end, wide-field fluorescent microscopy and a robotic device for motor control were combined for simultaneous recording of cortical neuronalactivity, force applied and forelimb position during task execution. Our results show that jRCaMP1a has sufficient sensitivity to monitor in vivo neuronal activity over multiple functional areas, and can be successfully used to perform longitudinal imaging in awake mice.